1. Field of the Invention
The present invention relates to an automatic focusing device for a photographing optical system, and more particularly to a control device for an automatic focusing device for forming an optical image of an object on photosensors having a plurality of photoelectric elements and focusing an optical system to the object in response to the output signals from the photosensors.
2. Description of the Prior Art
There are already known various automatic focusing devices in which an optical image of an object is formed on photosensors having a plurality of photoelectric elements and a photographing lens is focused to the object in response to the processed output signals of the photosensors. However, as long as the object image is photoelectrically detected by divided photosensors, any device encounters the same difficulty as in the manual focusing with human eyes for achieving an exact focusing in case the object has a low contrast. Consequently, if the focus position cannot be detected, a lens driving apparatus continuously reciprocates the photographing lens between an infinite focused position and a nearest focused position, thus wasting time and energy of a power source. In order to overcome such defect, it has been proposed to provide a timer circuit interlockingly connected to a main switch of the focusing device and to automatically turn off the lens driving apparatus at a time when a predetermined time passes after such reciprocating movement is made. Such device is, however, still insufficient for avoiding the above-mentioned waste of time and energy. Also in such device, in which the focus detecting operation is automatically terminated at a determined time after the main switch is turned on, even if a condition of contrast is favorable, the operator cannot focus the camera continuously and may therefore lose an optimum moment of shutter releasing, particularly in case of following a moving object. Besides, such device is unsuitable for continuous photographing operation, for example, with a motor driven camera.
As mentioned above, a high contrast of the object is an essential condition for a focusing operation. In other words, a dark object provides only a limited light intensity, so that the photosensors are unable to provide a sufficiently high signal-to-noise ratio in the output signals. Consequently, the focus detecting function is significantly deteriorated to reduce detecting accuracy, and is eventually disabled completely in worst conditions. On the other hand, the focusing operation has to be continued for a certain period depending on the structure of a certain kind of focusing device since the image contrast on the photosensors is high or low respectively when the object is in focus or out of focus. However, for a dark object, the light intensity on the photosensors remains almost constant during the focusing operation, so that the lens driving apparatus should be immediately stopped at the initial period of the focus detecting operation if the photosensors are unable to provide output signals in a sufficiently high signal-to-noise ratio.